This invention relates to sheet transfer devices in general and more particularly relates to means for transferring a moving sheet to gripper means on an intermittently operated conveyor chain.
Corrugated board box blanks are often made by platen-type cutting and creasing presses of the type described in the A.F. Shields U.S. Pat. No. 3,042,398, issued July 3, 1962 for a Sheet Gripping Means for cutting and Creasing Press. In the device of the aforesaid U.S. Pat. No. 3,042,398, the box blanks are made by platen mounted die means from sheets while the latter are held by grippers on a transfer gripper bar that is stationary while cutting and creasing takes place. The gripper bar is one of a series of similar bars that are equally spaced along a closed loop chain that is intermittently operated so that each gripper bar is moved between and stops at discrete locations, being stopped initially at a pickup location where a sheet is fed to the grippers, then moving downstream to a working position where the sheet is positioned between opposed platens which are operated to cut and crease the sheet, then moving downstream and stopping at a delivery location where the sheet which has been transformed into a box blank is released from the grippers. Thereafter, the gripper bar is moved to one or more additional discrete locations until arriving again at the pickup station where another sheet is fed to the grippers on the gripper bar or slat. In the prior art, the grippers at the feeding station usually closed on the sheet while the feed slat was stopped. Because of this, the sheet was subjected to rapid acceleration and deceleration which became troublesome when the sheets were fed from a printer to the cutting and creasing press.
More particularly, in the prior art when corrugated sheets emerged from the printer they were received by a two-section conveyor belt device having pressure rollers to hold the sheets on the belt. The first conveyor belt section operates at a speed to convey sheets at the same speed as the printer. However, the speed of the second or downstream conveyor belt section is approximately 50% faster than the speed of the upstream conveyor section to increase the spacing between the trailing edge of one sheet and the leading edge of the sheet immediately upstream thereof. This increased spacing will be reduced when the leading sheet is stopped to be engaged by the grippers of the gripper bar that is stopped at the infeed position.
This two-belt, two-spaced conveyor arrangement often causes the sheets to skew randomly when accelerating from low to high speed. In addition, any skewing of the sheets entering the transfer section tends to increase as speed changes. While the leading edge of the sheet may be squared by stops at the infeed station, variation in lateral position of the sheet across the machine occurs during squaring, since at this time the sheet tends to pivot around the stop that is first contacted by the sheet.